The autonomic system controls physiological activities of the body and the imbalance of autonomic tone (sympathetic/parasympathetic balance) is related to many diseases and conditions, including sleep disorders, eating disorders, obesity, anorexia, gastrointestinal tract disorders, hypertension, coma and epilepsy. Increased sympathetic and decreased parasympathetic tone during heart failure have been shown to be associated with left ventricular dysfunction and increased mortality. Conversely, sympathetic inhibition, as well as parasympathetic activation, have been associated with reduced arrhythmia vulnerability following a myocardial infarction and are thought to be cardioprotective in nature. Direct electrical stimulation of parasympathetic nerves can activate the baroreflex, inducing a reduction of sympathetic nerve activity and reducing blood pressure by decreasing vascular resistance. Moreover, direct stimulation of the vagal parasympathetic fibers has been shown to reduce heart rate. In addition, some research indicates that chronic stimulation of the vagus nerve may be of protective myocardial benefit following cardiac ischemic insult.
Neural stimulation therapy, such as cardioprotective vagal stimulation following myocardial infarction (MI) has the potential to produce proarrhythmic effects. Vagal stimulation may shorten atrial effective refractory period (AERP) and increase dispersion of refractoriness, thereby increasing vulnerability to atrial fibrillation. Vulnerability to atrial fibrillation may be increased in patients with prior MI or dialted cardiomyopathy with mitral regurgitation, since stretching of atrial tissue may increase ectopy.